I've found a place i'm very interested in about 650m from BTS Ekkamai (37sq.m and a 2013 development) for ~3m - although there are a few other places further down the BTS line (Phra Khanong onwards) with (shoebox ~28sq.m) sized studios. Away from the BTS there is a lot for sale in Suapalai Park Asoke-Ratchada (MRT Rama 9) but I'm not a big fan of huge high rise condominiums.
My aunt owns a nice condo at the far end of Sukhumvit Soi 4 (no giggling at the back), she thinks that one of the old Saranjai Mansion condos down Sukhumvit 6 Alley could be good (they are available from ~2.7m upwards), but they look ancient to me, and although very close to Nana BTS the area isn't really my bag. But, she is a Thai and supposedly knows what to look for, but who knows